The present invention relates to a flame detector type of fire alarm with a first circuit which by photoelectric means and a band-pass filter receives the emission of a flame at least in the wavelength range of carbon dioxide and the wavelength range of the flickering of the flame and produces useful signals therefrom for an alarm means. The invention also relates to a radiation detector for a flame alarm containing a sensor element and a filter arranged forwardly thereof.
It is generally known that most flammable substances such as wood, petroleum, oil and hydrocarbons or carbohydrates--in short, organic materials--emit strongly in the wavelength ranges of approximately .lambda.=2.7 .mu.m and particularly at approximately .lambda.=4.4 .mu.m when they undergo combustion. Radiation emission takes place in line spectra and band spectra, the wavelength range 2.7 .mu.m being characteristic for both water and carbon dioxide and 4.3 .mu.m being a characteristic of only carbon dioxide. The article entitled "Fire Detection using Infrared Resonance Radiation", pages 55 to 60, FIG. 6 which appeared in the journal "Report of Fire Research Institute of Japan", Ser. No. 30 of December 1969 describes the circuit of an alarm which is sensitive to flame emission and temperature. This alarm is designed for the infrared range. However, it is not foolproof against false alarms. If spurious infrared radiation is present, e.g. radiators or ovens, whose thermal radiation is periodically interrupted by an intervening fan or the like in a particular rhythm, an undesired alarm signal can result although there is no fire or flame.
Flame alarms are subjected in practise to the most different types of spurious radiation, which can trigger a false alarm. From Swiss Pat. Nos. 509,633, 519,651, 536,066 and 558,577 there are known to the art flame alarms which make use of the different properties of flames, in order to differentiate a flame from spurious radiation, such as daylight, solar radiation or artificial light sources. For instance, there is employed the different flame properties, such as the irregular flickering and the resultant irregular intensity fluctuations of the flame radiation, or the special colour temperature or spectral composition of the flame radiation. Since certain spurious radiation however can contain radiation constituents with similar properties and such spurious radiation in practise frequently is more intense by several orders of magnitude than the flame radiation to be detected, such flame alarms therefore are not completely foolproof against false alarms and can not be set to the highest maximum sensitivity.
From French Pat. No. 2,151,148 and the mentioned article of K. Nakajima, appearing in the "Report of Fire Research Institute of Japan", it is known that the radiation of a flame predominantly consists of a narrow band intensity peak in the spectral range of the resonance of carbon dioxide at about 4.3-4.4 .mu.m, apart from an appreciably weaker wideband spectral region in the range of visible radiation and the near infrared. The carbon dioxide-resonance radiation occurs practically at flames which occur when there is combusted organic materials, however never occur or only with decreasing intensity in the case of spurious radiation. A flame alarm which evaluates, among other criterion, essentially the resonance radiation of carbon dioxide, therefore is appreciably more secure against the giving of false alarms and less prone to disturbance than flame alarms which evaluate ultraviolet radiation, visible light or near infrared.
However, what is disadvantageous with such flame alarms which evaluate the carbon dioxide-resonance radiation is that the therein employed radiation detectors are too wideband and passed other radiation parts or components. Conventional interference filters for 4.3 .mu.m possess, for instance, sidebands which are located in the near infrared or in the visible region, so that spurious radiation in such spectral range was likewise detected. Nakajima therefore used a filter which was specially developed by the United States firm, Optical Coating Laboratory. This special filter encompassed the carbon dioxide-resonance radiation, however for practical applications was still too wideband (3.9-5.2 .mu.m). In order to eliminate neighboring spurious radiation it is therefore necessary to use a special lead selenide photoelement developed by Santa Barbara Research Center which is capable of eliminating radiation having a wavelength greater than 4.3 .mu.m. What is here disadvantageous is that at standard or normal temperatures the carbon dioxide-resonance radiation already is located at the edge of the sensitivity descent, so that the flame radiation can not be fully utilized and the sensitivity of the flame alarm does not attain the optimum possible value.